The ULF concept was originally developed by SGP (Simmering-Graz-Pauker) in conjunction with Elin and Siemens but all three companies are now under the Siemens umbrella. There are over 300 of these cars in service in Vienna, but apart from a single batch that run in Oradea in Romania, which were in any case a batch to Vienna specification including paintwork, they are unique to Vienna.

It was one of the very early low-floor trams and still claims to have the lowest step height. The design has evolved and the original A (short) and B (long) class cars have developed into the A1 and B1 classes. The principal changes are from water-cooled to air-cooled traction motors, addition of air conditioning, modified passenger information displays and detail differences in lighting (the last being the most visible).

This collection shows the evolution from a prototype to the B1 class and gives an opportunity to illustrate the technology in these fascinating vehicles.